Stun mastery dont work correct

Stun Mastery's Hits against you Cannot be Critical Strikes if you've been Stunned Recently doesn't work properly, and even if stunned, a character can still receive critical hits.
In the video, I'm using an Ultimatum where monsters always attack with cold crits. I first equip and then remove an amulet that converts all cold damage to lightning. While I have this amulet on, I'm stunned as expected, but when I remove it, monsters continue to crit, despite the Mastery stun and the fact that I should be immune to crit.
https://youtu.be/bsVRt2BVGSU

seaman#4993, Stun Mastery's Hits against you Cannot be Critical Strikes if you've been Stunned Recently: fairly reliable uptime, requires caution as certain modifiers that guarantee crits (Ultimatum mods, Ursine Funerary Altar) + modifiers that can cause you to take cold damage (e.g. map mods, Eater Eldritch Altars, or Taste of Hate) may make it difficult to find an initial stunning hit.

I have initial stunning hit.

This is still an issue, dying to cold crits with cold taken as lightning greatwolf.

I ran some tests and noticed that it also depends on the monster type. For example, melee monkeys don't stun, while archers and other monsters can stun, although in Ultimate with the Deadly and Prismatic modifiers, all monsters should deal critical hits with cold damage. I also tried using Freeze Immunity, since it's one of the conditions for getting stunned by a cold crit according to the wiki, and it also didn't help.